The present invention relates to a method and an apparatus for measuring blood volume ejected by cardiac contraction, and a vital sign monitor using such method and apparatus.
In medical facilities, variation in the hemodynamics of a patient in an operating room, an intensive care unit (ICU), an emergency room, a dialysis treatment room or the like needs to be monitored continuously as long as possible.
Conventionally, monitoring of the variation in the hemodynamics of such a patient has been predominantly carried out by direct monitoring of a blood pressure.
In a living body, a cardiac output and a vascular resistance are regulated such that the blood pressure of the centrum is limited within a predetermined range.
Therefore, in order to know the variation in the hemodynamics of a patient at an early stage, it is not enough to only monitor the blood pressure directly, and there is a need to know cause of change in the blood pressure when the change in the blood pressure is observed. For this reason, there is a need to monitor a change in the cardiac output, in addition to monitoring the change in the blood pressure.
As methods of measuring the change in the cardiac output to monitor the variation in the hemodynamics of a patient, there are conventional methods such as a thermodilution method, a dye dilution method, an ultrasound method and the like, which will be described below.
First, the thermodilution method will be discussed. In this method, a Swan-Ganz catheter is inserted through a carotid artery, a predetermined amount of cold saline or cold glucose solution is introduced to a vena cava or a right atrium, and the cardiac output resulting from a temperature change is measured in a pulmonary artery.
Recently, another thermodilution method has been proposed in which blood is warmed through a catheter, and the cardiac output resulting from a temperature change is measured. According to this method, the cardiac output can be measured automatically at a regular time interval.
Next, the dye dilution method will be discussed. In this method, a predetermined amount of dye is introduced to a vein, and then the dye concentration is measured invasively or non-invasively in a part where the dye is uniformly diluted and thus the concentration becomes constant, so that the cardiac output is measured.
Next, the ultrasound method will be discussed. In this method, an inner diameter of an arterial blood vessel such as the descending aorta and the blood flow rate therein are measured transesophageally using ultrasound, and thus the cardiac output is measured.
The above-described conventional methods of measuring the cardiac output by monitoring the variation in the hemodynamics of a patient have the following problems:
The thermodilution method involves a problem in that the measurement is intermittent and continuous measurements are not allowed. Further, insertion of a catheter in the thermodilution method is highly invasive for a patient and involves a risk of infection and the like. Moreover, a skilled medical person is required to insert a catheter and to conduct the measurement. Recently, a method based on continuous measurements has also been developed in the thermodilution method, however, the method still requires insertion of a catheter, and the above-described problems in connection with the insertion of a catheter cannot be solved.
The dye dilution method also has a problem in that continuous measurements are impossible. Further, the method requires a skilled medical person in the measuring process.
The ultrasound method imposes a burden of stress on a patient because a transducer is attached transesophageally. Recently, a non-invasive measurement performed on the body surface has also been made available as a kind of the ultrasound method, but the continuous measurements are still impossible.
Considering the requirement for an aid of a medical person with advanced skill and the procedures being highly invasive for a patient, none of the methods described above can be conducted easily and continuously, and it is difficult to monitor the variation in the hemodynamics of a patient continuously at all times by these methods.